Brushless motor and manufacturing method thereof

ABSTRACT

A second ball bearing is installed from a first axial side toward a second axial side into an interior of a bearing holder of a rotor through an opening of the bearing holder, so that an outer race of the second ball bearing is press fitted to an inner peripheral portion of the interior of the bearing holder. A first ball bearing is inserted from the first axial side toward the second axial side into the interior of the bearing holder of the rotor through the opening of the bearing holder after the installing of the second ball bearing, so that an outer race of the first ball bearing is press fitted to the inner peripheral portion of the interior of the bearing holder and is axially spaced from the outer race of the second ball bearing.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by referenceJapanese Patent Application No. 2007-321940 filed on Dec. 13, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a brushless motor and a manufacturingmethod thereof.

2. Description of Related Art

In a previously known type of brushless motor, a rotor is rotatablysupported relative to a motor shaft through two ball bearings (see, forexample, Japanese Unexamined Patent Publication No. H08-047199).Japanese Unexamined Patent Publication No. H08-047199 discloses aspindle motor. In the case of Japanese Unexamined Patent Publication No.H08-047199, a generally cylindrical bearing holder is formed in a rotor,and two ball bearings are received in an interior of the bearing holdersuch that the ball bearings are spaced from each other in an axialdirection of a stationary shaft. The rotor is supported by the ballbearings in a rotatable manner relative the stationary shaft, which isheld by a bracket.

However, in the case of Japanese Unexamined Patent Publication No.H08-047199, the bearing holder is placed radially inward of the stator,around which windings (acting as heating elements) are wound. Thus, theheat from the stator can be easily conducted to the ball bearings, whichare received in the interior of the bearing holder. Thereby, thebearings need to be highly heat resistant, so that costs of thebrushless motor are disadvantageously increased.

Furthermore, in the case of Japanese Unexamined Patent Publication No.H08-047199, two large diameter portions (increased diameter portions)are provided at two axially opposed ends, respectively, of an inner boreof the bearing holder to hold the ball bearings, respectively. Thus, inorder to install the ball bearings in the same common direction, therotor needs to be inverted after installation of one of the ballbearings into the bearing holders prior to installation of the other oneof the ball bearings into the bearing holder. Therefore, the workefficiency is reduced, and the costs are increased.

SUMMARY OF THE INVENTION

The present invention addresses the above disadvantages. Thus, it is anobjective of the present invention to provide a brushless motor and amanufacturing method thereof, which enable cost reduction.

To achieve the objective of the present invention, there is provided abrushless motor, which includes a motor shaft, a rotor and acenterpiece. The motor shaft is stationary held and extends between afirst axial side and a second axial side. The rotor is rotatable aboutthe motor shaft and includes a generally cylindrical bearing holder,which is placed on a radially outer side of the motor shaft and extendsin an axial direction of the motor shaft. The centerpiece is placed onthe second axial side of the rotor and includes a motor shaft holder, anaxial wall and a connecting wall. The motor shaft holder is coaxial withthe bearing holder and securely holds the motor shaft. The axial wall isplaced radially outward of the motor shaft holder and of the bearingholder and is radially spaced from the bearing holder by a gap. Theaxial wall extends in the axial direction of the motor shaft, and anaxial extent of the axial wall at least partially overlaps with an axialextent of the bearing holder in the axial direction of the motor shaft.The connecting wall connects between the motor shaft holder and theaxial wall on the second axial side of the bearing holder. A pluralityof cooling air inlet openings axially penetrates through the connectingwall in the axial direction of the motor shaft. The first ball bearingis received in an interior of the bearing holder and has an inner race,which receives the motor shaft, and an outer race, which is held by aninner peripheral portion of the interior of the bearing holder. Thesecond ball bearing is received in the interior of the bearing holderand is axially spaced from the first ball bearing on the second axialside thereof. The second ball bearing has an inner race, which receivesthe motor shaft, and an outer race, which is held by the innerperipheral portion of the interior of the bearing holder. The stator issecurely held by the axial wall of the centerpiece and generates arotational magnetic field to rotate the rotor upon energization of thestator.

To achieve the objective of the present invention, there is alsoprovided a manufacturing method of a brushless motor. According to themanufacturing method, a second ball bearing is installed from a firstaxial side toward a second axial side into an interior of a bearingholder of a rotor through an opening of the bearing holder, so that anouter race of the second ball bearing is press fitted to an innerperipheral portion of the interior of the bearing holder. Furthermore, afirst ball bearing is installed from the first axial side toward thesecond axial side into the interior of the bearing holder of the rotorthrough the opening of the bearing holder after the installing of thesecond ball bearing, so that an outer race of the first ball bearing ispress fitted to the inner peripheral portion of the interior of thebearing holder and is axially spaced from the outer race of the secondball bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objectives, features andadvantages thereof, will be best understood from the followingdescription, the appended claims and the accompanying drawings in which:

FIG. 1 is a plan view of a brushless motor according to an embodiment ofthe present invention;

FIG. 2 is a side view of the brushless motor of the embodiment;

FIG. 3 is a bottom view of the brushless motor of the embodiment;

FIG. 4 is a partial enlarged cross sectional view of the brushless motorof the embodiment;

FIG. 5 is a partial enlarged cross sectional view of the brushless motorof the embodiment;

FIG. 6 is a partial cross sectional plan view of the brushless motor ofthe embodiment;

FIG. 7 is a bottom view of a centerpiece of the brushless motor of theembodiment;

FIGS. 8A to 8E are schematic diagrams showing an assembling procedure ofthe brushless motor of the embodiment;

FIG. 9 is a partial perspective view showing an assembling work of thebrushless motor of the embodiment;

FIG. 10 is a partial cross sectional view showing the assembling work ofthe brushless motor of the embodiment;

FIG. 11 is a partial cross sectional view showing the assembling work ofthe brushless motor of the embodiment;

FIG. 12 is a schematic cross sectional view showing a modification ofthe brushless motor of the embodiment;

FIG. 13 is a schematic cross sectional view showing another modificationof the brushless motor of the embodiment;

FIG. 14 is a partial enlarged cross sectional view of a firstcomparative example of the brushless motor;

FIGS. 15A to 15F are schematic diagrams showing an assembling procedureof the first comparative example of the brushless motor;

FIG. 16 is a partial enlarged cross sectional view of a secondcomparative example of the brushless motor; and

FIGS. 17A to 17F are schematic diagrams showing an assembling procedureof the second comparative example of the brushless motor.

DETAILED DESCRIPTION OF THE INVENTION

A brushless motor according to an embodiment of the present inventionwill be described with reference to the accompanying drawings.

FIGS. 1 to 7 show a structure of the brushless motor 10 of the presentembodiment. The brushless motor 10 is suitable as, for example, a fanmotor of a vehicle (e.g., an automobile), which is used to cool aradiator of the vehicle. The brushless motor 10 includes a motor shaft12, a rotor 14, a centerpiece 16, a stator 18, a control unit 20 and aheat sink 22.

As shown in FIG. 4, the rotor 14 includes a rotor housing 24 and aplurality of rotor magnets 26. The rotor housing 24 is made of metal(e.g., iron) and is integrally formed into a generally cylindricalcup-shaped body through a die casting process, a press working processor the like. Specifically, the rotor housing 24 includes a generallycylindrical portion 28 and a bottom portion 30. The magnets 26 are fixedto an inner peripheral surface of the cylindrical portion 28 such thatthe magnets 26 are arranged one after another in a circumferentialdirection of the cylindrical portion 28. A blower fan 32 is integrallyrotatably installed to the rotor housing 24. A plurality of cooling airoutlet openings 34 is formed in the bottom portion 30 of the rotorhousing 24.

At a center part of the bottom portion 30 of the rotor housing 24, agenerally cylindrical bearing holder 36 extends in an axial direction ofthe motor shaft 12 at a location radially outward of the motor shaft 12.The bearing holder 36 has an opening 38 at one axial side (Z1 side) ofthe motor shaft 12 and a bottom portion 40 at the other axial side (Z2side) of the motor shaft 12. The one axial side (the Z1 side) and theother axial side (the Z2 side) may also be referred to as a first axialside and a second axial side, respectively.

As shown in FIG. 5, the bottom portion 40 supports an outer race 46B ofa second ball bearing 46, which will be described below in detail, fromthe other axial side (the Z2 side). A through hole 42 extends through acenter part of the bottom portion 40 in the axial direction of the motorshaft 12. A first ball bearing 44, the second ball bearing 46, a bush 48and a spring (e.g., a coil spring) 50 are received in the interior ofthe bearing holder 36.

The first ball bearing 44 includes an inner race 44A and an outer race44B. One end portion of the motor shaft 12 on the one axial side (the Z1side) is received through the inner race 44A, and the outer race 44B ispress fitted to an inner peripheral surface (inner peripheral portion)of the bearing holder 36.

The second ball bearing 46 is spaced from the first ball bearing 44 onthe other axial side (the Z2 side) of the motor shaft 12 and includes aninner race 46A and the outer race 46B. The other end portion of themotor shaft 12 on the other axial side (the Z2 side) is received throughthe inner race 46A, and the outer race 46B is press fitted to the innerperipheral surface of the bearing holder 36.

The bush 48 is received in the bearing holder 36 to support the innerrace 46A of the second ball bearing 46 from the one axial side (the Z1side) of the motor shaft 12. Furthermore, the motor shaft 12 is pressfitted into the bush 48.

The spring 50 is interposed between the bush 48 and the first ballbearing 44 and urges the inner race 44A of the first ball bearing 44away from the inner race 46A of the second ball bearing 46 toward theone axial side (the Z1 side). As indicated by an arrow P in FIG. 5, apre-load is applied to the inner race 44A of the first ball bearing 44and the outer race 46B of the second ball bearing 46 toward the oneaxial side (the Z1 side) of the motor shaft 12.

Furthermore, the interior, i.e., the inner bore of the bearing holder 36has a generally constant cross sectional area in the axial direction ofthe motor shaft 12. Specifically, an inner diameter of the interior ofthe bearing holder 36 is generally constant from the opening 38 to thebottom portion 40. Thereby, the bearing holder 36 permits installationof the first ball bearing 44, the second ball bearing 46, the bush 48and the spring 50 into the interior of the bearing holder 36 through theopening 38 of the bearing holder 36 from the one axial side (the Z1side) of the motor shaft 12.

The centerpiece 16 is made of metal (e.g., iron) and is integrallyformed through, for example, a die casting process. The centerpiece 16is placed on the other axial side (the Z2 side) of the motor shaft 12relative to the rotor 14, as shown in FIG. 4. The centerpiece 16includes a generally planar main body 52, which extends in a radialdirection of the motor shaft 12. A generally cylindrical motor shaftholder 54 projects from a center part of the main body 52 toward the oneaxial side (the Z1 side).

A holding hole 56 is formed in the motor shaft holder 54 and extendscoaxially with the through hole 42. The other end portion of the motorshaft 12 on the other axial end side (the Z2 side) is press fitted intoand is held in the holding hole 56. As shown in FIG. 5, an end part ofthe motor shaft holder 54, which is located on the one axial side (theZ1 side), supports the inner race 46A of the second ball bearing 46through the through hole 42 from the other axial side (the Z2 side).

Furthermore, the main body 52 has an axial wall 58, which surrounds thebearing holder 36 on a radially outward of the bearing holder 36 andaxially projects toward the one axial side (the Z1 side). The axial wall58 forms a gap 60 relative to the bearing holder 36. Also, the axialwall 58 is axially overlapped with a part of the bearing holder 36 onthe other axial end side (the Z2 side) of the motor shaft 12, that is,an axial extent of the axial wall 58 partially overlaps with an axialextent of the bearing holder 36 (thereby, the axial wall 58 having anoverlapping length L).

Furthermore, the main body 52 has a connecting wall 62, which connectsbetween a part of the motor shaft holder 54 and a part of the axial wall58 on the other axial side (the Z2 side) of the bearing holder 36. Aplurality (three in this instance) of cooling air inlet holes 64 extendsthrough the connecting wall 62 in the axial direction of the motor shaft12. The cooling air inlet holes 64 are arranged one after another atgenerally equal intervals in the circumferential direction of the motorshaft 12. The cooling air inlet holes 64 are provided at locations,which overlap with the bottom portion 40 in the radial direction of themotor shaft 12. That is, a radial extent of each of the cooling airinlet openings 64 at least partially overlaps with a radial extent ofthe bottom portion 40 of the bearing holder 36 in the radial directionof the motor shaft 12. In other words, at least a portion of eachcooling air inlet hole 64 is aligned with the bottom portion 40 in adirection parallel to the axial direction of the motor shaft 12.

Furthermore, as shown in FIG. 4, the main body 52 has a peripheralportion 66 on an outer side of the connecting wall 62 in the radialdirection of the motor shaft 12. The peripheral portion 66 has aplurality of winding cooling holes 68, which extend through theperipheral portion 66 in the axial direction of the motor shaft 12 andare arranged one after another in the circumferential direction of themotor shaft 12. The winding cooling holes 68 are placed at locations,which overlap with the corresponding slots 80, respectively, of a statorcore 70 (described below in greater detail) in the radial direction ofthe motor shaft 12. That is, at least a portion of each winding coolinghole 68 is aligned with the corresponding slot 80 in a directionparallel to the axial direction of the motor shaft 12.

The stator 18 includes the stator core 70, an insulator 72 and windings74. The stator core 70 is configured into an annular body and has aninner peripheral portion. When the inner peripheral portion of thestator core 70 is press fitted to an outer peripheral portion of theaxial wall 58, the stator core 70 is assembled integrally with thecenterpiece 16. A communication passage 76 is defined between the statorcore 70 and the shaft holding portion 36 in the radial direction of themotor shaft 12 and is communicated with the gap 60.

The communication passage 76 extends in the axial direction of the motorshaft 12 and is opened at the opposite side of the communication passage76, which is axially opposite from the gap 60, toward the one axial side(the Z1 side) in the stator 18.

The windings 74 are wound around the corresponding teeth 78 of thestator core 70 through the insulator 72 and are electrically connectedto a control circuit 82 described below. The stator 18 generates arotational magnetic field upon energization of the windings 74 to rotatethe rotor 14 according to a control signal received from the controlcircuit 82.

The control unit 20 includes the control circuit 82 and a unit case 84.As shown in FIG. 2, the heat sink 22 is installed integrally to thecontrol circuit 82. Furthermore, as shown in FIGS. 3 and 4, a pluralityof cooling air inlet openings 86 is formed in the unit case 84.

Next, the manufacturing (assembling) of the brushless motor 10 havingthe above structure will be described.

FIGS. 8A to 8E show an assembling procedure of the brushless motor 10according to the embodiment of the present invention.

First, as shown in FIG. 8A, the stator 18 is installed to the axial wall58 from the one axial side (the Z1 side), so that the stator 18 is heldby the axial wall 58.

Next, as shown in FIG. 8B, the motor shaft 12 is installed to the motorshaft holder 54 from the one axial side (the Z1 side) and is therebyheld in the motor shaft holder 54.

As shown in FIG. 8C, the second ball bearing 46 is inserted into theinterior of the bearing holder 36 through the opening 38 from the oneaxial side (the Z1 side), so that the outer race 46B of the second ballbearing 46 is held by the inner peripheral surface of the bearing holder36. Furthermore, the motor shaft 12 is inserted through the inner race46A of the second ball bearing 46 from the other axial side (the Z2side) until the inner race 46A contacts the motor shaft holder 54.

Next, as shown in FIG. 8D, the bush 48 and the spring 50 are receivedinto the interior of the bearing holder 36 through the opening 38 fromthe one axial side (the Z1 side). Also, at this time, the motor shaft 12is press fitted to the inner peripheral surface of the bush 48. The bush48 and the spring 50 serve as an urging means.

As shown in FIG. 8E, the first ball bearing 44 is inserted into theinterior of the bearing holder 36 through the opening 38 from the oneaxial side (the Z1 side), so that the outer race 44B of the first ballbearing 44 is held by the inner peripheral surface of the bearing holder36. Furthermore, the one end portion of the motor shaft 12 on the oneaxial side (the Z1 side) is inserted into the inner race 44A of thefirst ball bearing 44.

Here, FIGS. 9 to 11 show details of this assembling step of the firstball bearing. Specifically, as shown in FIGS. 9 and 10, in theassembling step of the first ball bearing, three projections 88 a of ajig 88 are axially inserted into the three cooling air inlet holes 64,respectively, from the other axial side (the Z2 side), so that distalends of the projections 88 a of the jig 88 contact the bottom portion 40to support the bottom portion 40 from the other axial side (the Z2side).

As shown in FIGS. 10 and 11, in the state where the projections 88 a ofthe jig 88 axially support the bottom portion 40 from the other axialside (the Z2 side), the first ball bearing 44 is inserted into theinterior of the bearing holder 36 through the opening 38 from the oneaxial side (the Z1 side). Thereby, the outer race 44B of the first ballbearing 44 is held by the inner peripheral surface of the bearing holder36, and the one end portion of the motor shaft 12 on the one axial side(the Z1 side) is inserted into the inner race 44A of the first ballbearing 44.

As discussed above, according to the embodiment, in the assembling stepof the first ball bearing 44, the jig 88 is used to insert the motorshaft 12 into the inner race 44A of the first ball bearing 44 at thetime of press fitting the first ball bearing 44 into the bearing holder36, so that, for example, deformation of the other part (e.g., thebottom portion 30) of the rotor 14 can be advantageously limited.

Next, the control unit 20 and the heat sink 22, which are shown in FIGS.1 to 4, are installed to the centerpiece 16. In this way, the assemblingof the brushless motor 10 of the present embodiment is completed.

The present embodiment provides the following advantages.

In the brushless motor 10 of the present embodiment, the first ballbearing 44 and the second ball bearing 46 can be easily installed intothe interior of the bearing holder 36 through the opening 38 from thecommon side, i.e., from the one axial side (the Z1 side). In this way,it is not required to invert the rotor 14 at the time of installing thefirst ball bearing 44 and the second ball bearing 46 to the bearingholder 36. Therefore, the work efficiency can be improved, and the costscan be reduced.

Also, in the brushless motor 10 of the present embodiment, theprojections 88 a of the jig 88 are inserted into the correspondingcooling air inlet holes 64, respectively, from the other axial side (theZ2 side) such that the distal ends of the projections 88 a of the jig 88contact the bottom portion 40. In this way, the bottom portion 40 can beaxially supported with the projections 88 a of the jig 88 from the otheraxial side (the Z2 side). Also, in this state, the first ball bearing 44is inserted into the interior of the bearing holder 36 through theopening 38 from the one axial side (the Z1 side), so that the outer race44B of the first ball bearing 44 is held by the inner peripheral surfaceof the bearing holder 36, and the one end portion of the motor shaft 12on the one axial side (the Z1 side) is inserted into the inner race 44Aof the first ball bearing 44.

In this way, it is possible to improve the workability at the time ofinserting the first ball bearing 44 into the bearing holder 36.Therefore, the work efficiency can be improved, and the costs can bereduced.

Furthermore, in the brushless motor 10 of the present embodiment, thebush 48 and the spring 50 are installed into the interior of the bearingholder 36 through the opening 38 from the one axial side (the Z1 side)in the direction, which is the common direction that is the same as theinstallation direction of the first ball bearing 44 and the second ballbearing 46 into the interior of the bearing holder 36. Therefore, it isnot required to invert the rotor 14 in order to install the bush 48 andthe spring 50 into the bearing holder 36. In this way, the workefficiency can be improved, and the costs can be reduced.

As discussed above, the brushless motor 10 of the present embodimentenables the cost reduction, so that it is possible to provide theinexpensive vehicle fan motor to the market.

Now, in order to provide the clear understanding of the advantages ofthe present embodiment, comparative examples will be described.

FIG. 14 shows a cross sectional view of a brushless motor 110 of a firstcomparative example, and FIGS. 15A to 15F show an assembling procedureof the brushless motor 110 of the first comparative example.Furthermore, FIG. 16 shows a cross sectional view of a brushless motor210 of a second comparative example, and FIGS. 17A to 17F show anassembling procedure of the brushless motor 210 of the secondcomparative example.

As shown in FIG. 14, the brushless motor 110 of the first comparativeexample has a generally cylindrical bearing holder 136, which isprovided at a center part of a centerpiece 116 and axially extendstoward the one axial side (the Z1 side). The bearing holder 136 issecurely press fitted into an axial hole 171, which is formed in astator core 170. In this way, the centerpiece 116 and the stator core170 are integrally fixed.

The bearing holder 136 has a bottom portion 140 at the one axial side(the Z1 side) and an opening 138 at the other axial side (the Z2 side).A first ball bearing 144, a bush 148, a second ball bearing 146 and awave washer 147 are installed into an interior of the bearing holder 136in this order from the one axial side (the Z1 side) toward the otheraxial side (the Z2 side). The wave washer 147 axially urges the secondball bearing 146 away from a support plate 149 toward the one axial side(the Z1 side).

The motor shaft 112 is supported by the first ball bearing 144 and thesecond ball bearing 146 in a rotatable manner relative to thecenterpiece 116. A rotor 114 is fixed to the one end portion of themotor shaft 112 on the one axial side (the Z1 side) to rotate integrallywith the motor shaft 112.

The brushless motor 110 of the first comparative example is assembled inthe assembling order shown in FIGS. 15A to 15F.

As shown in FIG. 15A, the centerpiece 116 is inverted to place theopening 138 toward the one axial side (the Z1 side). In this state, thefirst ball bearing 144 and the bush 148 are installed into the interiorof the bearing holder 136 from the one axial side (the Z1 side) throughthe opening 138.

Next, as shown in FIG. 15B, the centerpiece 116 is inverted to place theopening 138 toward the other axial side (the Z2 side). In this state,the stator 118 is installed into the bearing holder 136 from the oneaxial side (the Z1 side), so that the stator 118 is held by the bearingholder 136.

Then, as shown in FIG. 15C, in the state where the rotor 114 is fixed tothe one end portion of the motor shaft 112 on the one axial side (the Z1side), the motor shaft 112 is installed into the first ball bearing 144from the one axial side (the Z1 side) and is fixed.

As shown in FIG. 15D, the centerpiece 116 is inverted once again toplace the opening 138 toward the one axial side (the Z1 side). In thisstate, the second ball bearing 146 is installed into the interior of thebearing holder 136 from the one axial side (the Z1 side) through theopening 138.

As shown in FIG. 15E, the wave washer 147 is installed into the interiorof the bearing holder 136 from the one axial side (the Z1 side) throughthe opening 138. Furthermore, as shown in FIG. 15F, the support plate149 is installed to the centerpiece 16 from the one axial side (the Z1side). Thereby, the assembling of the brushless motor 110 of the firstcomparative example is completed.

In the case of the brushless motor 110 of the first comparative example,the rotor 114 needs to be inverted multiple times in order to installthe first ball bearing 144 and the second ball bearing 146 into thebearing holder 136 from the same direction. Therefore, the workefficiency is reduced, and the costs are increased.

Also, in the brushless motor 110 of the first comparative example, thebearing holder 136 is placed radially inward of the stator 118, aroundwhich the windings (heating elements) are wound. Therefore, the heat canbe easily transmitted to the first ball bearing 144 and the second ballbearing 146, which are received in the bearing holder 136, from thestator 118. Therefore, the first ball bearing 144 and the second ballbearing 146 need to be highly heat resistant ball bearings (e.g., ballbearings, to which heat resistant grease is applied or is heat treated),so that the costs are disadvantageously increased.

Furthermore, in the brushless motor 110 of the first comparativeexample, the rotor 114 is held only at the one end portion of the motorshaft 112 on the one axial side (the Z1 side), so that the length ofholding portion of the motor shaft 112, which holds the rotor 114,becomes relatively short. Thereby, the rotor 114 can be easily tiltedrelative to the motor shaft 112, so that the unbalance of the rotor 114may possibly occur. As a result, an adjustment work for compensating theunbalance of the rotor 114 is required, so that the costs aredisadvantageously increased.

Furthermore, as shown in FIG. 16, in the brushless motor 210 of thesecond comparative example, two large diameter portions (increaseddiameter portions) 237A, 237B are formed at two axial sides,respectively, of an inner bore of a bearing holder 236. The first ballbearing 244 and the second ball bearing 246 are held in the largediameter portions 237A, 237B, respectively. The bearing holder 236 has afirst opening 238A and a second opening 238B at two opposed axial ends,respectively, of the bearing holder 236.

The brushless motor 210 of the second comparative example is assembledin the assembling order shown in FIGS. 17A-17F First, as shown in FIG.17A, the stator 218 is installed to the axial wall 258 from the oneaxial side (the Z1 side), so that the stator 218 is held by the axialwall 258.

Next, as shown in FIG. 17B, a motor shaft 212 is installed to the motorshaft holder 254 from the one axial side (the Z1 side) and is therebyheld in the motor shaft holder 254.

As shown in FIG. 17C, a rotor 214 is inverted to place a second opening238B toward the one axial side (the Z1 side). In this state, the secondball bearing 246 is installed into the interior of the large diameterportion 237B of the bearing holder 236 from the one axial side (the Z1side) through the second opening 238B.

Next, as shown in FIG. 17D, the rotor 214 is inverted to place thesecond opening 238B toward the other axial side (the Z2 side). In thisstate, the motor shaft 212 is installed through the inner race of thesecond ball bearing 246 from the other axial side (the Z2 side) untilthe second ball bearing 246 contacts the axial wall 258.

Next, as shown in FIG. 17E, a bush 248 and a spring 250 are receivedinto the interior of the bearing holder 236 through the first opening238A from the one axial side (the Z1 side).

As shown in FIG. 17F, the first ball bearing 244 is installed into theinterior of the large diameter portion 237A of the bearing holder 236through the first opening 238A from the one axial side (the Z1 side),and the portion of the motor shaft 212 at the one axial side (the Z1side) is inserted through the inner race of the first ball bearing 244.Thereby, the assembling of the brushless motor 210 of the secondcomparative example is completed.

In the brushless motor 210 of the second comparative example, the twolarge diameter portions 237A, 237B are formed at the two axial sides,respectively, of the inner bore of the bearing holder 236. The firstball bearing 244 and the second ball bearing 246 are held in the largediameter portions 237A, 237B, respectively. Therefore, in order toinstall the first ball bearing 244 and the second ball bearing 246 intothe bearing holder 236 in the common direction, the rotor 214 needs tobe inverted after the installation of the second ball bearing 246 intothe bearing holder 236 prior to the installation of the first ballbearing 244 into the bearing holder 236. Therefore, the work efficiencyis reduced, and the costs are increased.

Also, in the brushless motor 210 of the second comparative example, thebearing holder 236 is placed radially inward of the stator 218, aroundwhich the windings (serving as the heating elements) are wound.Therefore, the heat can be easily transmitted to the first ball bearing244 and the second ball bearing 246, which are received in the bearingholder 236, from the stator 218. Therefore, the first ball bearing 244and the second ball bearing 246 need to be highly heat resistant,thereby resulting in the increased costs.

Unlike the above first and second comparative examples, the brushlessmotor 10 of the present embodiment does not require the inverting of therotor 14 at the time of installing the first ball bearing 44 and thesecond ball bearing 46 into the bearing holder 36. Therefore, the workefficiency can be improved, and the costs can be reduced.

Furthermore, in the brushless motor 10 of the present embodiment, thecooling air inlet holes 64 axially penetrate through the connecting wall62, which connects between the motor shaft holder 54 and the axial wall58 at the other axial side (the Z2 side) of the motor shaft holder 54and the axial wall 58. Therefore, as indicated by arrows A in FIG. 4,when the blower fan 32 is rotated, the cooling air is introduced intothe motor interior from the cooling air inlet openings 86 through thecooling air inlet holes 64, and this introduced cooling air can beapplied to the bearing holder 36.

Furthermore, the axial wall 58 is provided radially outward of thebearing holder 36 while the gap 60 is interposed between the axial wall58 and the bearing holder 36. The axial wall 58 extends in the axialdirection of the motor shaft 12 such that the axial wall 58 overlapswith the part of the bearing holder 36, which is located on the otheraxial side (the Z2 side), in the axial direction of the motor shaft 12.Therefore, the cooling air, which is introduced through the cooling airinlet holes 64, can be guided by the axial wall 58 along the outerperipheral surface of the bearing holder 36.

In this way, the first ball bearing 44 and the second ball bearing 46,which are received in the bearing holder 36, can be effectively cooled,so that it is not required to use the highly heat resistant ballbearings as the first ball bearing 44 and the second ball bearing 46.Thereby, the costs can be reduced.

Furthermore, in the brushless motor 10 of the present embodiment, asindicated by the arrows A in FIG. 4, the cooling air, which isintroduced through the cooling air inlet holes 64, can be discharged tothe side of the stator 18 on the one axial side (the Z1 side) from thegap 60 through the communication passage 76. In this way, the outerperipheral surface of the bearing holder 36 can be exposed to thecooling air along the axial direction. Thereby, the first ball bearing44 and the second ball bearing 46, which are received in the interior ofthe bearing holder 36, can be further cooled.

Furthermore, in the brushless motor 10 of the present embodiment, thefirst ball bearing 44 and the second ball bearing 46 are received in thebearing holder 36 such that the first ball bearing 44 and the secondball bearing 46 are spaced from each other in the axial direction of themotor shaft 12. Therefore, it is possible to have the relatively longdistance between the first ball bearing 44 and the second ball bearing46, which support the rotor 14 in the rotatable manner relative to themotor shaft 12. As a result, the more desirable rotational balance ofthe rotor 14 relative to the motor shaft 12 can be achieved. Thus, it ispossible to abolish the adjustment work for compensating the unbalanceof the rotor 14, so that the costs can be reduced.

The brushless motor 10 of the present embodiment provides the followingadvantages besides the above described advantages over the abovecomparative examples. That is, the winding cooling holes 68 are formedin the main body 52 of the centerpiece 16, so that the cooling air canbe introduced into the motor interior through the winding cooling holes68. Furthermore, this introduced cooling air can be discharged throughthe cooling air outlet openings 34 after passing through the slots 80.Therefore, the windings 74, which are wound at the slots 80, can be alsocooled with the cooling air.

Also, in the brushless motor 10 of the present embodiment, the innerperipheral surface of the stator 18 is fitted to the outer peripheralsurface of the axial wall 58. Thus, for example, as indicated by arrowsB in FIG. 4, even when the water intrudes into the motor interiorthrough the axial gap between the rotor 14 and the centerpiece 16, thefurther intrusion of the water toward the radially inside of the axialwall 58 can be limited. Thereby, it is possible to limit the applicationof the water to the first ball bearing 44 and the second ball bearing46. As a result, it is not required to use water resistant ball bearings(e.g., seal type ball bearings, which use a contact rubber) as the firstball bearing 44 and the second ball bearing 46, thereby allowing thecost reduction. Also, the sliding loss of the first ball bearing 44 andthe second ball bearing 46 can be reduced.

Furthermore, in the brushless motor 10 of the present embodiment, thebottom portion 30 of the rotor housing 24 and a bottom portion 33 of theblower fan 32 are tightly engaged with each other. Thereby, it ispossible to limit the intrusion of the water into the motor interiorthrough the gap between the bottom portion 30 of the rotor housing 24and the bottom portion 33 of the blower fan 32.

In the brushless motor 10 of the present embodiment, the inner race 46Aof the second ball bearing 46 can be supported by the end of the motorshaft holder 54, which is located on the one axial side (the Z1 side),from the other axial side (the Z2 side). Furthermore, the inner race 44Aof the first ball bearing 44 can be urged by the spring 50 away from theinner race 46A of the second ball bearing 46 toward the one axial side(the Z1 side).

Thereby, the pre-load can be applied to the inner race 44A of the firstball bearing 44 and the outer race 46B of the second ball bearing 46.Thus, it is possible to limit the looseness (rattling, jouncing) of theinner race and the outer race in each of the first ball bearing 44 andthe second ball bearing 46. In this way, it is possible to increase thelifetime of the first ball bearing 44 and the second ball bearing 46.

Furthermore, in the brushless motor 10 of the present embodiment, thesecond ball bearing 46 is supported by the bush 48 and the motor shaftholder 54 from the opposite axial sides, respectively. Therefore, evenwhen the relatively large vibrations are applied to the second ballbearing 46 through the motor shaft 12, application of an excessive forceto the spring 50, which applies the pre-load, can be limited. In thisway, the reliability of the spring 50 can be increased.

The embodiment of the present invention has been described above.However, the present invention is not limited to the above embodiment,and the above embodiment may be modified in various ways withoutdeparting from the spirit and scope of the present invention.

For example, in the above embodiment, the axial wall 58 is constructedto overlap with the part of the bearing holder 36, which is located onthe other axial side (the Z2 side), in the axial direction of the motorshaft 12. Alternatively, the axial wall 58 may be overlapped with theentire axial extent of the bearing holder 36 in the axial direction ofthe motor shaft 12.

For example, in the above embodiment, as shown in FIG. 5, the motorshaft holder 54 projects from the center part of the main body 52 towardthe one axial side (the Z1 side). This may be modified as follows.

That is, in a modification shown in FIG. 12, the motor shaft holder 54projects from the center part of the main body 52 toward the other axialside (the Z2 side).

Also, a washer (stopper) 55 is interposed between the motor shaft holder54 and the inner race of the second ball bearing 46. The part of themotor shaft holder 54, which is located on the one axial side (the Z1side), supports the inner race 46A of the second ball bearing 46 fromthe other axial side (the Z2 side) through the washer 55.

With this construction, a distance L2 from the first ball bearing 44 tothe main body 52 of the centerpiece 16 in this modification can be madeshorter in comparison to a distance L1 from the first ball bearing 44 tothe main body 52 of the centerpiece 16 in the case of FIG. 5 (i.e.,L2<L1). Thus, even when a bending force is applied from the rotor 14 tothe motor shaft 12 through the first ball bearing 44, a bending momentapplied to the main body 52 of the centerpiece 16 and the motor shaftholder 54 can be limited to a relatively small value. In this way,wobbling of the motor shaft 12 can be limited.

In this modification, the washer 55 is used. In place of the washer 55,a flange, which is similar to the washer 55, may be formed in the motorshaft 12 through, for example, a cold forging process.

Furthermore, in the above embodiment, the brushless motor 10 is used asthe fan motor of the vehicle. Alternatively, the brushless motor 10 maybe used in any other applications.

In the above embodiment, the stator 18 is installed to the axial wall 58of the centerpiece 16 before the installation of the motor shaft 12 tothe motor shaft holder 54. Alternatively, the motor shaft 12 may beinstalled to the motor shaft holder 54 before the installation of thestator 18 to the axial wall 58. Furthermore, in the above embodiment,the second ball bearing 46 is installed into the bearing holder 36 afterthe installation of the motor shaft 12 to the motor shaft holder 54.Alternatively, the second ball bearing 46 may be installed to the motorshaft 12 and then to the bearing holder 36 before the installation ofthe motor shaft 12 to the motor shaft holder 54. Furthermore, theprojections 88 a of the jig 88 may be previously installed through thecooling air inlet openings 64 of the connecting wall 62 of thecenterpiece 16 before the installation of the rotor 14.

In the above embodiment, the centerpiece 16 is integrally formed throughthe die casting process. Alternatively, as shown in FIG. 13, thecenterpiece 16 may be integrally formed through a press working processof a metal plate material. In the case of the modification shown in FIG.13, the axial wall 58 and the motor shaft holder 54 are both formed bybending corresponding portions of the metal plate material. Unlike theabove embodiment, the connecting wall 62 is placed on the one axial side(the Z1 side) of the motor shaft holder 54 at a location adjacent to thebottom portion 40 of the bearing holder 36. In order to limit directcontact between the bottom portion 40 of the bearing holder 36 and theconnecting wall 62, a stopper 12 a is formed integrally with the motorshaft 12 through, for example, a cold forging process, and is interposedbetween the inner race 46A of the second ball bearing 46 and the axialend of the motor shaft holder 54, which is placed adjacent to and isformed integrally with the connecting wall 62. Thus, the bearing holder36 of the rotor 14 is effectively spaced from the connecting wall 62 ofthe centerpiece 16 by a predetermined axial gap to enable smoothrotation of the rotor 14. In this modification, the stopper 12 a isformed integrally with the motor shaft 12. Alternatively, similar to thewasher 55 of FIG. 12, the stopper 12 a may be formed separately from themotor shaft 12 and may be installed to the motor shaft 12. Furthermore,in FIG. 13, the motor shaft holder 54 may be further axially extendedtoward a location where a center of mass of the rotor 14 is located. Inthis way, the rotation of the rotor 14 may be further stabilized.

In the above embodiment as well as the modifications thereof, the coilspring 50 is placed around the motor shaft 12. In place of the coilspring, a Belleville spring may be placed around the motor shaft 12.

Additional advantages and modifications will readily occur to thoseskilled in the art. The invention in its broader terms is therefore notlimited to the specific details, representative apparatus, andillustrative examples shown and described.

1. A brushless motor comprising: a motor shaft that is stationary heldand extends between a first axial side and a second axial side; a rotorthat is rotatable about the motor shaft and includes a generallycylindrical bearing holder, which is placed on a radially outer side ofthe motor shaft and extends in an axial direction of the motor shaft; acenterpiece that is placed on the second axial side of the rotor andincludes: a motor shaft holder that is coaxial with the bearing holderand securely holds the motor shaft; an axial wall that is placedradially outward of the motor shaft holder and of the bearing holder andis radially spaced from the bearing holder by a gap, wherein the axialwall extends in the axial direction of the motor shaft, and an axialextent of the axial wall at least partially overlaps with an axialextent of the bearing holder in the axial direction of the motor shaft;and a connecting wall that connects between the motor shaft holder andthe axial wall on the second axial side of the bearing holder, wherein aplurality of cooling air inlet openings axially penetrates through theconnecting wall in the axial direction of the motor shaft; a first ballbearing that is received in an interior of the bearing holder and has aninner race, which receives the motor shaft, and an outer race, which isheld by an inner peripheral portion of the interior of the bearingholder; a second ball bearing that is received in the interior of thebearing holder and is axially spaced from the first ball bearing on thesecond axial side thereof, wherein the second ball bearing has an innerrace, which receives the motor shaft, and an outer race, which is heldby the inner peripheral portion of the interior of the bearing holder;and a stator that is securely held by the axial wall of the centerpieceand generates a rotational magnetic field to rotate the rotor uponenergization of the stator.
 2. The brushless motor according to claim 1,wherein: the stator defines a communication passage between the statorand the bearing holder in the radial direction of the motor shaft; andthe communication passage is communicated with the gap on the secondaxial side thereof and opens in the axial direction of motor shaft onthe first axial side thereof.
 3. The brushless motor according to claim1, wherein: the bearing holder has an opening, which opens in the axialdirection of the motor shaft on the first axial side; and the bearingholder is configured to receive the first ball bearing and the secondball bearing into the interior of the bearing holder through the openingof the bearing holder from the first axial side.
 4. The brushless motoraccording to claim 3, wherein: the bearing holder has a bottom portionon the second axial side thereof; the bottom portion of the bearingholder axially supports the outer race of the second ball bearing fromthe second axial side thereof; and a radial extent of each of theplurality of cooling air inlet openings at least partially overlaps witha radial extent of the bottom portion of the bearing holder in theradial direction of the motor shaft.
 5. The brushless motor according toclaim 1, wherein: the bearing holder has: an opening, which opens in theaxial direction of the motor shaft on the first axial side; and a bottomportion, which is located on the second axial side to axially supportthe outer race of the second ball bearing from the second axial sidethereof; a through hole axially extends through the bottom portion inthe axial direction of the motor shaft and is coaxial with the motorshaft holder; the motor shaft holder axially supports the inner race ofthe second ball bearing from the second axial side thereof through thethrough hole; and the brushless motor further comprises an urging meansfor urging the inner race of the first ball bearing away the inner raceof the second ball bearing in the axial direction of the motor shafttoward the first axial side.
 6. The brushless motor according to claim5, wherein the urging means includes: a bush that is received in theinterior of the bearing holder and is held by the motor shaft, whereinthe bush has an inner peripheral portion that is held by the motorshaft, and the bush axially supports the inner race of the second ballbearing from the first axial side thereof; and a spring that isinterposed between the bush and the first ball bearing.
 7. The brushlessmotor according to claim 1, further comprising a blower fan, which isintegrally rotatably fixed to the rotor, and the brushless motor servesas a fan motor of a vehicle.
 8. The brushless motor according to claim1, wherein the interior of the bearing holder has a generally constantinner diameter along a generally entire axial extent of the interior ofthe bearing holder.
 9. A manufacturing method of a brushless motor,comprising: installing a second ball bearing from a first axial sidetoward a second axial side into an interior of a bearing holder of arotor through an opening of the bearing holder, so that an outer race ofthe second ball bearing is press fitted to an inner peripheral portionof the interior of the bearing holder; and installing a first ballbearing from the first axial side toward the second axial side into theinterior of the bearing holder of the rotor through the opening of thebearing holder after the installing of the second ball bearing, so thatan outer race of the first ball bearing is press fitted to the innerperipheral portion of the interior of the bearing holder and is axiallyspaced from the outer race of the second ball bearing.
 10. Themanufacturing method according to claim 9, further comprising installinga stator, which generates a rotational magnetic field to rotate therotor upon energization of the stator, from the first axial side towardthe second axial side such that the stator is installed to and issecurely held by an axial wall of a centerpiece.
 11. The manufacturingmethod according to claim 10, further comprising installing a motorshaft from the first axial side toward the second axial side such thatthe motor shaft is installed into and is securely held by a motor shaftholder of the centerpiece, which is placed radially inward of the axialwall and is connected to the axial wall by a connecting wall.
 12. Themanufacturing method according to claim 11, further comprisinginstalling the rotor from the first axial side toward the second axialside over the motor shaft.
 13. The manufacturing method according toclaim 12, wherein the installing of the second ball baring into theinterior of the bearing holder is executed before or after theinstalling of the rotor.
 14. The manufacturing method according to claim13, wherein: the installing of the second ball bearing into the interiorof the bearing holder is executed before the installing of the rotor;the installing of the rotor includes receiving the motor shaft throughan inner race of the second ball bearing, which is held by the bearingholder of the rotor; and the installing of the first ball bearing intothe interior of the bearing hole is executed after the installing of therotor.
 15. The manufacturing method according to claim 11, furthercomprising contacting a plurality of projections of a jig to a bottomportion of the bearing holder and axially supports the bottom portion ofthe bearing holder on the second axial side thereof prior to theinstalling of the first ball bearing.
 16. The manufacturing methodaccording to claim 15, wherein the contacting of the plurality ofprojects of the jig to the bottom portion of the bearing holder includesinserting the plurality of projections of the jig through a plurality ofcooling air inlet openings, respectively, which penetrate through theconnecting wall in the axial direction of the motor shaft to place theplurality of projections of the jig in position before making thecontacting of the plurality of projections to the bottom portion of thebearing holder.
 17. The manufacturing method according to claim 9,further comprising installing an urging means from the first axial sidetoward the second axial side into the interior of the bearing holderthrough the opening of the bearing holder after the installing of thesecond ball bearing and before the installing of the first ball bearing.18. The manufacturing method according to claim 17, wherein theinstalling of the urging means includes installing a spring as theurging means from the first axial side toward the second axial sidearound the motor shaft.